Lyme disease is the leading vector-borne bacterial disease in the United States and probably the world. The etiologic agent of this disease, Borrelia burgdorferi, is transmitted by the globally distributed ixodid ticks of the genus Ixodes. In addition to the acute form of the illness, a chronic infection may develop that can persist for years. The early and late manifestation of Lyme disease can be correlated with the presence of low numbers of viable spirochetes. The most effective antimicrobial, dosage, route and length of treatment have not been established for Lyme disease. Although clinical studies suggest that most patients respond satisfactorily to antimicrobial therapy if initiated early in the disease, exceptions to this occur particularly when patients have neurologic involvement. However, evaluation of the treatment of this disease is problematic due to the spirochetes latency and the intermittent pattern of exacerbations and remissions in the natural history of untreated infections. Standard methods for detemining the in vitro and in vivo antimicrobials susceptibility of fastidious, slow-growing bacteria, such as B. burgdorferi with a generation time of 10-20 hr have not been establdhed. It is important to have these methods available to identify antimicrobials that could be effective for the treatment of Lyme disease and an animal model to investigate various treatment protocols. The specific aims of this proposal are: I. Develop standard method for determination of in vitro antimicrobial susceptibility of B. burgdorferi. Following development of standard in vitro antimicrobial susceptibility protocol: A. Determine sensitivity of B. burgdorferi to new as well as some of the commonly used antimicrobials and antimicrobial combinations. B. Compare in vitro susceptibility of B. burgdorferi isolates from diverse geographical areas to representative antimicrobial agents. II. Develop standard method for determination of in vivo antimicrobial susceptibility of B. burgdorferi in experimentally infected hamsters. Following development of standard method for the in vivo antimicrobial susceptibility assay: A. Determine in vivo susceptibility of B. burgdorferi to antimicrobials that possess good in vitro activity. B. Compare efficacy of combined antimicrobial therapy with single agent therapy. C. Evaluate effect of steroids on antimicrobial therapy. D. Compare efficacy of antimicrobial therapy in hamsters experimentally infected by injection versus infection via tick bite. E. Evaluate effectiveness of antimicrobial prophylaxis for tick transmitted B. burgdorferi infection.